Estimate Fetal Weight (EFW)
Enter ultrasound biometric measurements in millimeters (mm). Choose a formula, then click calculate.
What this fetal weight estimation calculator does
This tool estimates fetal weight from standard second- and third-trimester ultrasound measurements. It uses established Hadlock equations, which combine biometric values such as abdominal circumference (AC), femur length (FL), head circumference (HC), and biparietal diameter (BPD) to produce an estimated fetal weight (EFW) in grams.
In practice, EFW helps clinicians track growth trends over time rather than relying on a single number. A repeated pattern of growth can be more informative than one isolated scan value.
Inputs explained
Abdominal circumference (AC)
AC is one of the strongest contributors to fetal weight estimation because it reflects liver size, soft tissue, and overall growth.
Femur length (FL)
FL approximates longitudinal skeletal growth. It is used in many formulas and helps improve weight estimation when paired with AC.
Head circumference (HC) and biparietal diameter (BPD)
HC and BPD provide head-size context. Including these values (in the 4-parameter Hadlock formula) can refine the estimate, especially when body proportions vary.
Formulas used in this calculator
| Formula | Equation |
|---|---|
| Hadlock (AC + FL) | log10(EFW) = 1.304 + 0.05281×AC + 0.1938×FL − 0.004×AC×FL |
| Hadlock (BPD + HC + AC + FL) | log10(EFW) = 1.3596 − 0.00386×AC×FL + 0.0064×HC + 0.00061×BPD×AC + 0.0424×AC + 0.174×FL |
In both equations above, measurements are converted to centimeters before calculation. Output is returned in grams and in pounds/ounces.
How to interpret the result
- EFW is an estimate, not exact birth weight. Typical uncertainty can be around ±10% (sometimes more).
- Trend matters most. Serial growth scans are often more meaningful than a single EFW.
- Clinical context matters. Placenta, amniotic fluid, Dopplers, maternal factors, and gestational age all affect interpretation.
Why estimated fetal weight can differ from actual birth weight
Measurement variability
Small differences in caliper placement can shift AC, HC, or FL enough to change EFW materially.
Fetal position
Suboptimal fetal position can make anatomic landmarks harder to capture, which increases variability.
Population-level formulas applied to individuals
Hadlock equations were derived from large groups. Any individual fetus can naturally deviate from formula assumptions.
Frequently asked questions
Is a low estimated fetal weight always dangerous?
Not always. Some fetuses are constitutionally small but healthy. Your clinician may correlate with growth percentiles, Doppler studies, and maternal-fetal history.
Is a high estimated fetal weight always a reason for intervention?
Not necessarily. Management depends on many factors including diabetes status, pelvic assessment, prior obstetric history, and the full clinical picture.
Can I use this as a substitute for prenatal care?
No. This calculator is for educational use only and should not replace professional obstetric evaluation.
Bottom line
A fetal weight estimation calculator is useful for understanding how ultrasound measurements map to estimated fetal weight. Use it to learn and to follow discussions with your care team, but let your obstetric provider make diagnostic and treatment decisions.